Anisotropic mold liner for continuous casting of metals



Jan 28, 1969 Y J. w. WARREN, JR

ANISOTROPIC MOLD LINER FOR CONTINUOUS'CASTING OF METALS Filed April 8, 1966 Ill/11111111111 James W. War/e, J:

INVENTOR. WHANN 8 McMA/V/GAI. Af/omcy: for Anal/knot United States Patent 6 Claims ABSTRACT OF THE DISCLOSURE A mold having a portion for receiving hot metal and a rod or bar forming portion which consists of a body of pyrolytic graphite having an a-b plane of high heat conductivity, said pyrolytic graphite having a mold opening formed therein along an axis in a direction at right angles to the a-b plane so that the molten metal flowing therethrough contacts the pyrolytic graphite at the edges of the a-b plane and having a heat sink surrounding the pyrolytic graphite at the solidifying area of said mold, said heat sink being in heat transfer relationship to outer edges of the ab plane of said pyrolytic graphite body.

This invention relates generally to molds and particularly relates to a liner of anisotropic material for such molds adapted for continuously casting primary metals including metal alloys.

In recent years non-ferrous metals, including alloys, have been cast continuously to obtain, for example, rod, bar, plate or billet stock. This process has been used particularly for continuously casting copper and its alloys such as brass as well as other non-ferrous metals such as aluminum or magnesium. It has also been proposed to cast continuously iron and iron alloys such as steel.

Copper and its alloys and other non-ferrous metals are usually cast in a mold which may consist essentially of copper. The copper is conventionally lined with a material which serves as a lubricant. Usually the mold which is used for the continuous casting of non-ferrous metals is provided with a suitable heat sink such as a watercooled block of cooper.

.In the case of copper, for example, the casting temperature is around 2150 Fahrenheit (R), while copper solidifies at 1981 F. Thus, within a few inches of mold the temperature of the copper or copper alloy must be cooled approximately 200 F.

It has also been proposed to cast steel continuously in a mold. However, certain liquid metals such as steel have a tendency to wet the material of which the mold is made. Thus, steel has a tendency to stick to the mold as it solidifies. In order to overcome this problem it has been proposed to oscillate or reciprocate the mold. This, of course, provides a relative movement between the mold and the solidifying metal to prevent sticking of the metal to the mold. The steel is usually poured at a temperature of around 2430 F. and solidifies at around 2 150 F. Thus, in the case of steel the temperature of the metal must be reduced by almost 300 F. to cause it to solidify. Generally, all that is necessary is to solidify the outer skin of the steel which is sufiicient to prevent so-called breakouts where the liquid steel breaks through the solid surface layer to cause considerable damage.

The advantage of the continuous casting of metals (the term metal as used herein includes metal alloys) is the reduction of the number of steps required in treating the metal. In addition, there is a reduction of labor, a reduction in the quantity of material which is being treated which results in a reduced storage space, as well as a reduction of the time required and an increase of the 3,424,228 Patented Jan. 28, 1969 "Ice yield of metal. Thus, great economic gains can be obtained by continuously casting primary metals including both non-ferrous and ferrous metals and their alloys. However, there are certain drawbacks to the present methods of continuously casting metals.

The primary problem is the removal of heat from the cooling metal during the steady state of operation. This, of course, means that the heat must be transferred from the liquid metal into the mold or a suitable heat sink so that the metal solidifies rapidly to permit further handling. It will be apparent that the more rapidly the heat can be removed, the more rapidly the metal can be cast. Thus, one of the limitations of present techniques is the'relatively slow casting rate which necessitates the provision of several molds disposed in parallel and arranged to be fed from the same ladle or tundish.

The second problem that must be dealt with it is the wetting problem, that is, the fact that the liquid metal tends to wet the mold and, hence, stick to the mold. This is conventionally overcome as explained before by oscillating the mold and, of course, may result in an unsatisfactory surface finish.

Thus, what is required is a material for the mold or a suitable liner for the mold which has a high thermal conductivity, a high melting point so as to withstand the temperatures of liquid metals and a poor wettability by liquid metals.

In the past it has been proposed to utilize ordinary graphite for lining the mold. However, ordinary graphite is an isotropic material. This means that it conducts heat equally well (or badly) in all directions. The heat conductivity of copper of which the mold essentially consists, and which also constitutes the heat sink, is 25 times that of graphite.

In addition, graphite tends to erode mechanically and has a relatively high oxidation rate resulting in a very low useful life of a graphite mold or liner. Such molds generally have to be replaced within a day or week depending on the use.

Equally detrimental is the mechanical erosion of a 'graphite mold which is due primarily to the formation of dendritic crystals in the solidifying metal. Such dendrites are sharp and relatively hard, that is, much harder than graphite. Hence, the dendrites tend to remove mechanically the individual particles of which the graphite mold consists.

In spite of these drawbacks of present-day techniques for the continuous casting of non-ferrous metals, there has been an increasing tendency of the industry to utilize continous casting of non-ferrous metals in production and to acquire experience by pilot production of steel by the continuous casting process.

It is, accordingly, an object of the present invention to provide a mold for continuously casting metals including metal alloys which has a better heat conductivity than previously used materials but still prevents heat loss between the reservoir of liquid metal and the heat sink.

Another object of the present invention is to provide a liner for a mold for the continuous casting of metals which permits an increased casting rate due to the faster rate of cooling of the solidifying metal.

Still another object of the present invention is to provide a liner for a mold of the type referred to which reduces mechanical erosion, minimizes oxidation, acts as a lubricant and provides a better surface finish by reducing wetting between the metal and the mold.

In accordance with the present invention there is provided a mold for continuously casting metals. It should be noted that the term metal or metals is meant to include metal alloys such as brass, steel and the like. This mold has a portion which is disposed substantially between the area where the liquid metal is poured and the area where at least the outer surface of the metal has solidified.

In accordance with the present invention this portion of the mold consists of a refractory material which has anisotropic heat conducting properties. Preferably, such a metal consists of pyrolytic graphite although other materials may be used instead.

In addition, this refractory anisotropic material is oriented in a predetermined manner to accelerate cooling of the metal in the mold. Thus, preferably the refractory and anisotropic material is so oriented that it conducts heat relatively rapidly away from the metal across the wall of the mold while it conducts heat relatively slowly along the path of movement of the cooling metal of the mold. This latter, of course, serves the purpose to prevent the direct cooling of the reservoir of liquid metal by the heat sink.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawing, wherein the single figure is a schematic cross-sectional view through a mold embodying the present invention.

Referring now to the single figure of the drawing there is illustrated a mold generally shown at disposed below a suitable crucible, ladle or tundish 11. The ladle .11 contains the molten metal to be cast and is used to fill the mold 10 at a predetermined rate. There is further provided a refactory material shown at 12 which backs the mold proper as is followed :by a heat sink 1-4. The heat sink may, for example, consist of a block of copper provided with suitable ducts 15 for passing cooling water through the copper block in a convention manner.

In accordance with the present invention the mold is provided with a liner 20, adjacent the liquid metal and disposed at least between the area where the liquid metal is poured, that is, below the ladle 11 and the heat sink 14. In accordance with the present invention this portion of the mold or liner consists of a refractory material having anisotropic heat-conducting properties. Preferably, such a material has a ratio between the heat-conductivity in one plane and another plane at right angles thereto of 50 to 1. For example, pyrolytically deposited boron nitride (BN) is a suitable material which is refractory and has anisotropic heat-conducting properties as just defined. It is also feasible to utilize mica for this purpose. The presently preferred material for this purpose is, however, pyrolytic graphite.

Pyrolytic graphite is deposited from a vapor containing carbon at elevated temperatures in random layers which are disposed like disarranged stack of cards. This is the reason why pyrolytic graphite has highly anisotropic characteristics. Its mechanical, thermal and electrical properties depend upon the direction. It has become conventional practice to designate as a-b axes, which in turn refine a plane, those in which the graphite is deposited. The c-axis is at right angles to the a-b plane. Pyrolytic graphite conducts heat very well in the a-b plane 'but is highly insulating in the c-axis or direction. Thus, the heat conductivity of pyrolytic graphite is 250 times as great in the a-b plane as in the c-direction.

As explained above, pyrolytic graphite is deposited from a vapor which may be a chemical compound. This may, for example, be effected by dissociating methane (CH under the influence of heat. This is preferably done in a vacuum furnace at a pressure which may vary within a wide range but may, for example, be between about 1 and 10 mm. of mercury. The temperature of the furnace may also vary within a Wide range but preferably is around 2200 F. The manner of depositing pyrolytic graphite is, of course, well known in the art.

In accordance with the present invention, the liner 20 of refractory material having anisotropic heat-conducting properties is oriented in such a manner that it will conduct heat relatively rapidly away from the liquid metal within the space 21 and across the wall and into the heat sink 14, While at the same time conducting heat relatively slowly along the path of movement of the cooling metal in the mold. In other words, this prevents the heat of the molten metal from the ladle 11 and above the mold from being conducted away directly into the heat sink 14 rather than permitting the metal to cool slowly so that any crosssection has a relatively uniform temperature.

To this end the a-b plane of the anisotropic material such as pyrolytic graphite is oriented in the horizontal direction as indicated. Hence, the c-axis is disposed along the direction shown by the arrow 22. This will do precisely what is needed, namely, to prevent a rapid conduction of heat in the vertical direction as viewed in the drawing, while at the same time conducting heat in a horizontal direction into the heat sink 14.

It should be realized that during the continuous casting of steel, for example, about of the heat is removed while the metal passes the first 4" of the mold. Unless suflicient heat is removed in this manner the outer layer of the steel will be unable to cool and solidify to withstand the hydrostatic pressure of the steel within this solid shell. This, in turn, would cause breakouts whereby the liquid metal spills from the casting mold.

It should also be noted that the heat conductivity of pyrolytic graphite in the a-b plane is equivalent to or higher than that of copper depending on the temperature. On the other hand, pyrolytic graphite in the c-direction is practically an isulator of heat.

The oxidation rate of pyrolytic graphite at 1500 F. is 6500 times less than that of commercial graphite. Hence, by utilizing pyrolytic graphite the oxidation is tremendously reduced.

Furthermore, the structure of pyrolytic graphite is such that it is practically impervious in the c-direction. It has a density of 2.2 which is theoretical for this crystal structure. Commercial grades of graphite achieve densities of about 1.65. Therefore, since the much more dense and abrasion resistant layers of pyrolytic graphite are exposed to the erosive action of the molten metal, mechanical erosion is substantially reduced. Also pyrolytic material as distinguished from ordinary graphite does not contain individual particles which could be loosened or removed by the mechanical erosive action of the hot metal.

Accordingly, a mold in accordance with the present invention for continuously casting metals is provided with an anisotropic refractory material which is so oriented that it conducts heat relatively rapidly away from the metal toward the heat sink, while at the same time conducting heat relatively slowly along the path of movement of the cooling metal. This, as pointed out before, will cause the metal to have a temperature gradient across its cross-section which is more uniform than can be obtained with conventional molds. Further, since during steady state operation the heat is removed more efficiently than with conventional molds, the casting rate is increased due to the faster rate of cooling of the metal.

Finally, a mold in accordance with the present invention reduces erosion and minimizes oxidation. For this reason it provides, of course, a better surface finish by reducing wetting as well as erosion. Since the heat conductivity of pyrolytic graphite in the a-b plane is approximately equal to that of copper or may even be higher than that of copper, it matches the thermal properties of the watercooled copper block acting as a heat sink. This, of course, improves the removal of the heat during steady state operation.

The invention and its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the form, construction and arrangement of the parts of the invention without departing from the spirit and scope thereof or sacrificing its material advantages, the arrangement hereinbefore described merely by way of example and I do not wish to be restricted to the specific form shown or uses mentioned except as defined in the accompanying claims, wherein various portions have been separated for clarity of reading and not for emphasis.

I claim:

1. A mold for continuously casting metals, said mold having a portion disposed substantially between the area where the liquid metal is poured and the area where at least the outer surface of the metal has solidified,

(a) said portion consisting of a refractory material having anisotropic heat-conducting properties,

(b) said refractory material being oriented so that it conducts heat relatively rapidly away from the metal across the wall of the mold and conducts heat relatively slowly along the path of movement of the cooling metal in said mold, thus preventing loss of heat from said liquid metal area in a direction axially of said mold portion, and

(c) a heat sink at the lower end of said mold portion.

2. A mold as defined in claim 1 wherein said refractory material consists of pyrolytic graphite.

3. A mold as defined in claim 1 wherein said refractory material consists of pyrolytic graphite having its ab axes oriented at right angles to said path of movement and having its c-axis oriented substantially parallel to said path of movement.

4. A mold for continuously casting metals including metal alloys, said mold having a portion disposed substantially between the area where the liquid metal is poured and the area where at least the outer surface of the metal has solidified,

(a) said portion consisting of a refractory material having anisotropic heat-conducting properties,

(b) said refractory material being oriented in a predetermined manner, thereby to accelerate cooling of the metal in said mold.

5. A mold for continuously casting metals, said mold having (a) a part adapted to receive liquid metal;

(b) a mold portion extending therefrom into which the molten metal may flow, at least a portion of this mold portion consisting of anisotropic material having a mold opening formed in it on an axis substantially at right angles to the a-b planes of said anisotropic material; and

(c) a heat sink surrounding the portion of said anisotropic material where solidification is to occur, parts being related together in order that in the mold portion heat is allowed to flow outwardly in a direction transverse to the movement of metal through the mold to prevent heat loss from the liquid metal portion by restraining the flow of heat in the direction of movement of the metal through the mold, and to provide cooling in the part of said anisotropic material where solidification is to occur by direct high rate heat transfer to said heat sink.

6. The combination as defined in claim 5 in which molten metal is fed into said mold opening in a plane substantially at right angles to the a-b plane of said anisotropic material and in which the heat sink withdraws heat from the hot metal along the a-b plane of said anisotropic material in order to accomplish more rapid cooling of the metal in this part of said mold.

References Cited UNITED STATES PATENTS 3,096,083 7/1963 Keon 263-2 3,210,812 10/1965 Berwick 164-273 3,353,584- 11/1967 Atkin 164-283 X OTHER REFERENCES Pyrolytic Graphite-Engineering Handbook, General Electric Co.-1963.

I. SPENCER OVERHOLSER, Primary Examiner.

EUGENE MAR, Assistant Examiner.

U.S. Cl. X.R. 164283, 371 

